Plasma guns may be used, inter alia, for such purposes as thermal spraying. Thermal spraying involves the heat softening of a heat fusible material, such as a metal or ceramic, and propelling the softened material in particulate form against a substrate surface which is to be coated. The heated particles strike the surface and bond thereto. A conventional thermal spray gun such as a plasma gun is used for the purpose of both heating and propelling the particles. In a plasma spray gun, the heat fusible material is supplied to the gun in powder form, typically comprised of small particles: e.g., below 100 mesh U.S. standard screen size to about 5 microns.
In typical plasma systems an electrical arc is created between a water-cooled nozzle (anode) and an adjacently disposed cathode. A selected inert gas, flowing between the electrodes and through the electric arc, is ionized and heated to form a plasma attaining temperatures of up to 15,000 degrees Centigrade. The movement of the gas between the electrodes effectively lengthens the arc and causes more energy to be delivered to the arc. The plasma, constituted of at least partially ionized gas, issuing from the nozzle, resembles an open oxy-acetylene flame.
A plasma "flame" spray gun of the general type with which this invention is concerned is described in U.S. Pat. No. 3,145,287 issued on Aug. 18, 1964 to W. A. Siebein et al. for a "Plasma Flame Generator and Spray Gun". A more recent gun structure of this type is disclosed in U.S. patent application Ser. No. 646,734 filed on Sept. 4, 1984 by Anthony F. Dellassio et al. for a "Nozzle Assembly for Plasma Spray Gun" assigned to the same assignee as the present invention. The present invention may be implemented as a modification of the structures disclosed in the Seibein patent or the Dellassio application.
At this juncture it should be understood that "radial" and "tangential" are relative terms and, as used herein, "tangential" includes not only strictly tangential flow but also chordal flow, i.e., flow having a significant tangential component. Moreover, these terms are used in relation to the axis of a plasma flow path and/or the structure, e.g., a bore or conduit, which defines the path.
Plasma guns customarily are capable of operating with either argon or nitrogen as the primary plasma gas. For argon the gas is introduced into a chamber near the cathode with a tangential component so as to impart a vortical flow to the plasma as described, for example, in U.S. Pat. No. 3,823,302 issued July 9, 1974 to Muehlberger for "Apparatus and Method for Plasma Spraying". The reason for so doing is that, absent the vortex, the arc is not carried far enough down the nozzle, (i.e., not sufficiently lengthened by gas flow) to achieve the desired high arc voltage and efficiency.
On the other hand, radial gas flow input as described in the aforementioned U.S. Pat. No. 3,145,287 is generally used with nitrogen because it is less readily ionized and vortical flow with its tendency to extend the arc a long distance down the nozzle causes difficult starting of the arc.
However, without a vortex, the arc voltage and efficiency are low for nitrogen. Therefore, a secondary gas such as hydrogen is often added to the nitrogen, having the effect of facilitating the starting while permitting efficient operation without a vortex. The hydrogen is added after the arc is started. Controlling the hydrogen secondary gas necessarily entails complications and cost to the spraying operation as well as requiring special precautions against explosion.
Even with a vortex the efficiency for argon is undesirably low. Hydrogen is again resorted to as an additive where possible, but that gas is often considered undesirable because of its flammability and its causing embrittlement in the sprayed coating. Helium is an alternative but is expensive and less effective.
Generally, each plasma spray gun is set up for a particular type of plasma-forming gas, either with a radial or a tangential inlet. Guns that may be used for either primary gas typically have different gas distribution rings selectively inserted near the cathode for providing either radial or tangential flow; this requires disassembly when a change in gases is made. Several efforts have been made to simplify the change. U.S. Pat. No. 3,313,908 discloses a plasma torch with two types of gas inlet ports for different gases that are selected alternatively by means of either of two external gas conduit fittings. This method still requires changing those gun fittings and does not provide for adjusting the degree of vortical flow.
U.S. Pat. No. 3,851,140, issued Nov. 26, 1974 to Coucher for "Plasma Spray Gun and Method for Applying Coatings on a Substrate", shows a plasma spray gun with a gas distribution ring having primary openings slanted toward the axis of the gun and secondary openings tangentially oriented. The two sets of inlet openings function simultaneously. This ring is said to control alteration of the gas flow, but there is no means to alter the flow for different gases without changing rings, nor is there means to change the flow configuration during operation.
In U.S. Patent Reissue No. 25,088 reissued Nov. 21, 1961 to A. C. Ducati et al. for "Plasma-jet Torch Apparatus and Method Relating to Increasing the Life of the Back Electrode", there is depicted a plasma torch in which gas is introduced at two axially separated locations. Near the cathode a radial source is provided for the portion of the arc and associated plasma flowing from the cathode region through a first orifice. A tangential gas source is provided in a separate chamber region of large diameter downstream of the first orifice. These separated gas inlet sources do not provide for gas inlet control in the proximity of the cathode, and the anode and cathode are so widely spaced apart that the arc is very difficult to start. This problem is so serious that the arc must be started by momentarily inserting a conductor such as a piece of graphite between the electrodes. With such electrode separation, there is no way of easily starting the arc by changing gas mixtures or gas flow characteristics.
In view of the foregoing, one object of the present invention is to provide an improved plasma spray method and gun apparatus which can operate efficiently with nitrogen gas alone in a vortical flow, and which is not difficult to start.
Another object of the invention is to provide an easy-starting, high efficiency nitrogen gas plasma flame spray method and gun apparatus which avoids the need for the addition of hydrogen or other gases to the nitrogen in order to improve the starting characteristics.
Further objects and advantages of the invention will be apparent from the following description and the accompanying drawings. For instance, the invention permits the spray gun to be used either with argon or with nitrogen, using either with optimum efficiency and ease in starting.